I propose to use neurophysiological and psychophysical technqiues to examine several aspects of visual function in goldfish. Previous work shows that the psychophysically determined absolute threshold of goldfish and human rod systems are approximately the same; the goldfish may therefore be an appropriate model for threshold scotopic function in humans. The first objective is to define the nature of the response of optic tract neurons to stimuli that elicit behavioral responses at absolute threshold. By sampling many units and examining their patterns of response to such threshold stimuli, I hope to be able to establish a correlation between behavioral threshold and neurophysiological activity. This correlaion would provide the first behaviorally relevant definition of "threshold" in visual system neurons. Part of the research involves simultaneous recording of neurophysiological and classically conditioned responses to presentation of the same visual stimuli. The second objective is to examine how changes in the degree of convergence of rods upon ganglion cells affects absolute threshold. Adult goldfish continue to grow, and retinal neurons continue to increase in number. Rods increase at a faster rate than the other elements, however, so the degree the convergence of rods upon more proximal elements is greater in large fish than in small fish. I propose to test absolute threshold in fish of different sizes to determine whether the increase in the ratio of rods to ganglion cells affects visual sensitivity. This is a psychophysical experiment, in which I will use fish who have been classically conditioned to change their respiratory rates in response to light. The third objective is to examine one aspect of neural plasticity in the goldfish retina, namely the apparent ability of photoreceptors to reorient so that the light incident upon them is parallel to their long axes. I propose to measure the photopic Stiles-Crawford effect (by means of the electroretinogram) following the application of an offset pupil for varying periods of time.